Measuring in-situ UV intensity in UV cure tool

ABSTRACT

Provided are improved apparatus and methods for radiative treatment. In some embodiments, a semiconductor processing apparatus for radiative cure includes a process chamber and a radiation assembly external to the process chamber. The radiation assembly transmits radiation into the chamber on a substrate holder through a chamber window. A radiation detector measures radiation intensity from time to time. The assembly includes a gas inlet and exhaust operable to flow a radiation-activatable cooling gas through the radiation assembly.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 12/008,149, filed Jan. 8, 2008, issued as U.S. Pat.No.7,935,940, which is incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

This invention relates to apparatus and methods for radiation treatment,including radiation treatment of thin films.

BACKGROUND

Many layers of thin films are used to make an integrated circuit. ICmanufacturing requires thin films to have certain properties in orderfor the circuit to function as designed. For example, there is a generalneed for materials with low dielectric constants (low-k). Using low-kmaterials as the intermetal dielectric (i.e., the layer of insulatorseparating consecutive levels of the conductive metal interconnects)reduces the delay in signal propagation due to capacitive effects,otherwise know as the RC delay. A dielectric material of low dielectricconstant will have low capacitance, and hence the RC delay of an ICconstructed with such a material will be lower as well.

As another example, there is a general need for materials with specifictensile or compressive stresses. Increasing shallow trench isolation(STI) film tensile stress increases transistor drain current and deviceperformance because the electron and hole mobilities are higher. Otherapplications require dielectric films to have compressive stress. Theseand other properties may be met on the film as deposited, or aftertreatment.

One such treatment may be a thermal process in which the substrate isheated to a temperature for a time. A thermal treatment may removeunwanted particles from the film, or change its stresses and otherproperties. These thermal processes, however, have certain difficulties.In particular, substrate temperatures generally need to be high (i.e.,greater than about 500 degrees Celsius) with exposure times typically onthe order of hours. As is well known in the field, these conditions candamage copper containing devices, especially in the application whereina low-k dielectric is being cured and the long exposure time may beunsuitable for mass manufacturing. Also, the use of temperaturesensitive nickel silicide precludes inducing film stress by usingtemperatures above 400° C. while some SiN films have a cure temperatureup to 480° C.

To overcome these disadvantages of thermal processing, another techniquehas been developed, which involves curing the film with UV radiation.Irradiation of the low-k or spacer nitride films permits modulation ofdesired film properties such as dielectric constant or film stress atlower temperatures. However, the use of UV radiation in such processesmay result in deleterious side-effects unless special care is taken todeliver the precise amount of radiation and to maintain the substratetemperature at an optimal level. Delivery of the precise amount ofradiation may be complicated when UV lamp intensity drifts over time andUV exposure results affect optical parts.

In other applications, radiation treatment may be performed usingelectromagnetic radiation from any part of the electromagnetic spectrum,including infra-red (IR), x-ray, and microwave radiation.

SUMMARY

Provided are improved apparatus and methods for radiative treatment. Insome embodiments, a semiconductor processing apparatus for radiativecure includes a process chamber and a radiation assembly external to theprocess chamber. The radiation assembly transmits radiation into thechamber on a substrate holder through a chamber window. A radiationdetector measures radiation intensity from time to time. The assemblyincludes a gas inlet and exhaust operable to flow aradiation-activatable cooling gas through the radiation assembly.

In one aspect, a semiconductor processing apparatus for UV cure includesa process chamber and a UV radiation assembly external to the processchamber. The UV radiation assembly transmits radiation into the chamberon a substrate holder through a chamber window. A UV detector measuresUV intensity from time to time. The assembly includes a gas inlet andexhaust operable to flow a UV-activatable cooling gas through theradiation assembly.

In another aspect, the present invention pertains to a UV apparatusincluding a process chamber and a UV radiation assembly mounted on theprocess chamber. The UV radiation assembly includes one or more UVradiation sources, one or more reflectors operable to direct a portionof the UV radiation through the window towards the substrate holder, anda UV intensity detector. This detector is mounted below the UV lamps andexternal to the chamber. It is oriented toward the window and detectsreflected light on a calibration substrate or a mirror placed above thewindow. A cover, e.g., a shutter or an iris, protects the detector fromUV radiation when not in use. The UV intensity detector is able towithstand high UV energy. The UV detector may be a silicon carbide (SiC)photodiode or photodetector. It may also include a diffuser. The UVdetector may also include a gas inlet and exhaust operable to flow aUV-activatable cooling gas through the radiation assembly.

The mirror is oriented toward the detector and reflects UV radiationtoward the detector. It may be detachable from the UV radiation assemblyor may be configured to not interfere with UV delivery to the chamberduring substrate processing. The mirror may be coated with a hightemperature UV coating or a metal coating, such as aluminum coating. Thecalibration substrate reflects UV light in the wavelengths of interest.The substrate may be a bare silicon wafer, a wafer coated with hightemperature UV coating or another appropriate coating such as aluminum.

The apparatus may also include a controller configured to execute a setof instructions. The controller compensates for measured intensity byexecuting instructions to measure a UV intensity at a UV radiationpower, calculate a deviation based on the measurement and a baselineintensity, and adjust the UV radiation power or exposure duration tocompensate for the deviation. The controller may also direct the coverto move, by executing instruction to open the cover, e.g., shutter oriris, to expose a UV detector to the UV radiation and close a shutter oriris to isolate the UV detector from the UV radiation.

In another aspect, the present invention pertains to methods ofcalibrating a UV apparatus. The method may be simple compensation ofmeasured UV intensity, issuance of alarm or a warning, or determinationof maintenance or corrective action. The UV apparatus calibration may beperformed while the pedestal remains hot and the chamber under vacuum,i.e., without shutting down or removing the system or chamber fromservice. The method includes receiving a calibration indication ordetermining that a calibration is required; positioning a calibrationsubstrate on a substrate holder; setting a UV radiation power to a firstpower; measuring a UV detector output over a duration; and, removing thecalibration substrate.

The calibration indication may be triggered in software by duration ofexposure, total intensity, time lapse, number of wafers processed, etcor be triggered manually by an operator. A calibration substrate isprovided to the chamber on a substrate holder. This calibrationsubstrate may be kept in a front opening unified pod (FOUP) in theplant's substrate warehouse system or in a storage locker. Thecalibration substrate may be a bare silicon wafer with certain UVreflectivity or be coated. Appropriate coatings are those that canwithstand periodic UV exposure with minimal change in UV reflectivity.Appropriate coatings may include high temperature UV coating anddeposited metal, such as aluminum.

After the calibration substrate is positioned on the substrate holder,it is exposed to UV radiation at a first power, which may be chosen bythe operator. The first power may be at 50%, 75% or 100% power. The UVradiation reflects off the calibration substrate back through the windowand is measured by the UV detector over a duration of time. At the endof the measurement duration, the UV power may be turned off, the UVdetector may be blocked from radiation, or the signal measurement maysimply stop. The calibration substrate may be removed after themeasurement duration.

The method may also include operations to open and close a cover thatprotects the UV detector from effects of continuous exposure to the UVradiation during normal processing. This cover may be actuated remotelythrough a controller to an open or a close position.

The method may also include calculating a UV intensity based on a the UVdetector output measured and calculating a deviation based on thecalculated UV intensity and a baseline intensity, and adjusting the UVradiation power or exposure time to compensate for the deviation. UVradiation generated by a lamp first passes through the chamber window tothe substrate, a portion of the radiation reflects off the substratethen back through the window before it is detected by the UV detector.Thus the radiation intensity detected at the detector is not the same asthat experienced by the substrate. A controller may be programmed totake into account the reflectivity of the substrate at various UVwavelengths to arrive at a UV intensity. This intensity only needs to becomparable from test to test. It may be a normalized value based on somemaximum, it may be an absolute value in watts per centimeter squared(W/cm²). In some cases, the raw detector output may be used asindication of intensity. This measured UV intensity is compared to adesired value and corrective action taken to compensate for a deviation.For example, if the measured UV intensity is lower than desired, than UVpower may be increased or exposure time may be increased if power cannotbe further increased. If the measured UV intensity is more than desired,than UV power and exposure time may be correspondingly adjusted.

In certain embodiments, the method may also include positioning a mirrorbetween a process chamber and the UV detector, measuring the reflectedUV radiation, and determining whether the process window between themirror and the substrate requires a cleaning. The difference betweenreflected UV radiation from the mirror and from the substrate is thepath. The reflected UV radiation from the substrate passes through thechamber window twice, whereas the UV radiation from the window does not.Knowing the reflectivity of the mirror and the substrate, the controllercalculates a difference that is attributable to the window. As discussedsome UV curing results in gaseous byproducts that can deposit on thewindow and obscure it. If the difference attributable to the window isgreater than a predetermined amount, then a signal to clean the window,either automatically through the system control or manually through analarm or warning to the operator, is generated.

Other required maintenance or corrective activity may be determined.This determination may be in the form of an alarm or warning to theoperator, or even logs associated with a batch of wafers processed.Particularly, the activity may be to replace UV lamps if the measured UVintensity too low to be compensated by lengthened process time. Inaddition to cleaning the window, the chamber itself may require cleaningor the detector may need to be replaced.

These and other features and advantages of the invention will bedescribed in more detail below with reference to the associateddrawings.

The apparatus may also include a controller configured to execute a setof instructions. The controller compensates for measured intensity byexecuting instructions to measure a UV intensity at a UV radiationpower, calculate a deviation based on the measurement and a baselineintensity, and adjust the UV radiation power or exposure duration tocompensate for the deviation. The controller may also direct the coverto move, by executing instruction to open the cover, e.g., shutter oriris, to expose a UV detector to the UV radiation and close a shutter oriris to isolate the UV detector from the UV radiation.

In another aspect, the present invention pertains to methods ofcalibrating a UV apparatus. The method may be simple compensation ofmeasured UV intensity, issuance of alarm or a warning, or determinationof maintenance or corrective action. The UV apparatus calibration may beperformed while the pedestal remains hot and the chamber under vacuum,i.e., without shutting down or removing the system or chamber fromservice. The method includes receiving a calibration indication ordetermining that a calibration is required; positioning a calibrationsubstrate on a substrate holder; setting a UV radiation power to a firstpower; measuring a UV detector output over a duration; and, removing thecalibration substrate.

The calibration indication may be triggered in software by duration ofexposure, total intensity, time lapse, number of wafers processed, etcor be triggered manually by an operator. A calibration substrate isprovided to the chamber on a substrate holder. This calibrationsubstrate may be kept in a front opening unified pod (FOUP) in theplant's substrate warehouse system or in a storage locker. Thecalibration substrate may be a bare silicon wafer with certain UVreflectivity or be coated. Appropriate coatings are those that canwithstand periodic UV exposure with minimal change in UV reflectivity.Appropriate coatings may include high temperature UV coating anddeposited metal, such as aluminum.

After the calibration substrate is positioned on the substrate holder,it is exposed to UV radiation at a first power, which may be chosen bythe operator. The first power may be at 50%, 75% or 100% power. The UVradiation reflects off the calibration substrate back through the windowand is measured by the UV detector over a duration of time. At the endof the measurement duration, the UV power may be turned off, the UVdetector may be blocked from radiation, or the signal measurement maysimply stop. The calibration substrate may be removed after themeasurement duration.

The method may also include operations to open and close a cover thatprotects the UV detector from effects of continuous exposure to the UVradiation during normal processing. This cover may be actuated remotelythrough a controller to an open or a close position.

The method may also include calculating a UV intensity based on a the UVdetector output measured and calculating a deviation based on thecalculated UV intensity and a baseline intensity, and adjusting the UVradiation power or exposure time to compensate for the deviation. UVradiation generated by a lamp first passes through the chamber window tothe substrate, a portion of the radiation reflects off the substratethen back through the window before it is detected by the UV detector.Thus the radiation intensity detected at the detector is not the same asthat experienced by the substrate. A controller may be programmed totake into account the reflectivity of the substrate at various UVwavelengths to arrive at a UV intensity. This intensity only needs to becomparable from test to test. It may be a normalized value based on somemaximum, it may be an absolute value in watts per centimeter squared(W/cm²). In some cases, the raw detector output may be used asindication of intensity. This measured UV intensity is compared to adesired value and corrective action taken to compensate for a deviation.For example, if the measured UV intensity is lower than desired, than UVpower may be increased or exposure time may be increased if power cannotbe further increased. If the measured UV intensity is more than desired,than UV power and exposure time may be correspondingly adjusted.

In certain embodiments, the method may also include positioning a mirrorbetween a process chamber and the UV detector, measuring the reflectedUV radiation, and determining whether the process window between themirror and the substrate requires a cleaning. The difference betweenreflected UV radiation from the mirror and from the substrate is thepath. The reflected UV radiation from the substrate passes through thechamber window twice, whereas the UV radiation from the window does not.Knowing the reflectivity of the mirror and the substrate, the controllercalculates a difference that is attributable to the window. As discussedsome UV curing results in gaseous byproducts that can deposit on thewindow and obscure it. If the difference attributable to the window isgreater than a predetermined amount, then a signal to clean the window,either automatically through the system control or manually through analarm or warning to the operator, is generated.

Other required maintenance or corrective activity may be determined.This determination may be in the form of an alarm or warning to theoperator, or even logs associated with a batch of wafers processed.Particularly, the activity may be to replace UV lamps if the measured UVintensity too low to be compensated by lengthened process time. Inaddition to cleaning the window, the chamber itself may require cleaningor the detector may need to be replaced.

These and other features and advantages of the invention will bedescribed in more detail below with reference to the associateddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1D are schematics of UV lamp assembly with in-situ UVdetector assembly and mirror in various positions.

FIGS. 2A and 2B are process flow diagrams depicting a process inaccordance with certain embodiments of the present invention.

FIGS. 3A-3D are schematics of UV detector assemblies in accordance withcertain embodiments of the present invention

FIG. 4 is a cross-section schematic of a UV lamp assembly and chamber inaccordance with certain embodiments of the present invention.

FIGS. 5A and 5B are schematics of a multi-station UV cure chamber inaccordance with certain embodiments of the present invention.

FIG. 6 is a graph showing detected UV signal % over the course of usageof a lamp, comparing a clean N₂-only coolant gas and an ozone-generatingcoolant gas.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Introduction

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Thepresent invention may be practiced without some or all of these specificdetails. In other instances, well known process operations have not beendescribed in detail to not unnecessarily obscure the present invention.While the invention will be described in conjunction with the specificembodiments, it will be understood that it is not intended to limit theinvention to the embodiments.

Reference will be made in detail to implementations of the presentinvention as illustrated in the accompanying drawings. The samereference indicators will be used throughout the drawings and thefollowing detailed description to refer to the same or like parts. Inthis application, the terms “work piece,” “wafer” and “substrate” willbe used interchangeably. The following detailed description assumes theinvention is implemented on a wafer. However, the invention is not solimited. The work piece may be of various shapes, sizes, and materials(e.g., displays of various sizes).

The present invention involves an ultraviolet (UV) irradiation apparatusfor processing semiconductor substrates. UV radiation is used to treatthin films on substrates to achieve various property changes. UVradiation can break chemical bonds, change composition of material,cause chemical reactions to occur, affect density and stress, andotherwise provide energy to a substrate.

A typical implementation of UV irradiation apparatus has a number of UVlamps to illuminate a substrate. The UV lamps may be flood lamps,generating UV light in a broad range of wavelengths, or a high intensitydischarge light that emits UV radiation in a smaller spectrum.Monochromatic sources of UV radiation, such as certain lasers, may alsobe used. Lamps may be powered through excitation by electrodes ormicrowave energy and may be inductively coupled.

In certain implementations, two individual linear mercury bulbs are usedin two individual lamp assemblies. With a two-bulb configuration, therelative orientation of the bulbs to the circular substrate is variedover time to achieve a uniform exposure for each circular section.Various reflectors are also used in lamp assemblies to direct generatedUV radiation toward the substrate. Linear bulbs generate light in alldirections, but the substrate surface is only on one side of the bulb.Thus reflectors are used to direct UV radiation that would haveotherwise escape from the lamp assembly toward the substrate. This way,lamp power is used more efficiently. In other implementations, the bulbsmay be of different shapes, e.g., light bulb shape or toroidal.

In order to achieve the various film property changes on a substrate,the amount of curing that occurs as result of exposure to UV radiationmust correspond to the amount of property changed desired. Thus for someproperty changes, a small amount of cure is required and for others, alarger amount of cure is required. The amount of UV cure is a functionof UV radiation intensity, substrate temperature, and in some casespressure and the gas present in the chamber. To ensure that the amountof film property change is constant from one substrate to another, theamount of cure must be controlled over time to ensure wafer to waferuniformity.

Reliable methods are known to control substrate temperature, chamberpressure, and the gas present in the chamber. The substrate may beheated or cooled during the UV exposure. This heating or cooling mayoccur by controlling the temperature of the pedestal on which thesubstrate is positioned or by adding gas into the chamber at a certaintemperature. Chamber pressure is controlled through vacuum pumps orcommon vacuum forelines and pressure gauges. The gas in the chamber maybe controlled by first evacuating the chamber then adding whichever gasis desired. During some UV treatments, the gas in the chamber mayactively participate in the changing of film properties by reacting withthe film.

UV radiation intensity is also controlled. UV radiation intensity asexperienced by the substrate depends on a number of factors. The powerapplied to the UV lamps determines how much radiation is generated atthe source. The condition of the UV lamps determines how effective thepower is used to generate UV radiation. The type of cooling gas in theUV lamp assembly determines how much radiation is absorbed. The designof the reflectors and the reflectivity determines how much radiation isdirected through the chamber window. The type of material used for thechamber window and its cleanliness determines how much of the radiationdirected at the window passes through. Finally the radiation reaches thesubstrate at a certain intensity.

The contribution of these factors changes over time in part becausevarious parts of the UV lamp assembly and the chamber may becomesolarized over time. Solarization refers to a phenomenon where amaterial undergoes a change after being subjected to high energyelectromagnetic radiation, such as ultraviolet light. Clear glass andmany plastics will turn blue and/or degrade after long term solarexposure in the desert. It is believed that solarization is caused bythe formation of internal defects, called color centers, whichselectively absorb portions of the visible light spectrum. Solarizationmay also permanently degrade a material's physical or mechanicalproperties, and is one of the mechanisms involved in the breakdown ofplastics within the environment. Prolonged exposure to high intensity UVlight such as that used in UV cure damages chamber parts, especiallythose made from organic polymers and affects reflectivity over time.Some materials may be more resistant to UV-induced damage, such asTeflon, PTFE and PFA

Two major factors that affect UV intensity over time at the substrateare the condition of UV bulbs and the chamber window. UV lamp bulbs usedfor curing are subjected to high powers, e.g., for intensities up to 1W/cm^2, and sustain high cycling. The bulb performance degrades overtime. Given the same power input, the output intensity reduces overtime. In order to maintain the same intensity output, bulbs may besubjected initially to a lower power that is slowly increased over timeto compensate for the decrease in performance. At some point, the bulbsare replaced. The bulbs may be replaced on certain triggering eventsregardless of their performance. These events may be a number of cycles,total cure time, total time, the number of substrates cured, totalpower, etc. The bulbs may also be replaced based on performance, forexample, if the bulb does not output an expected intensity. Efficientuse of bulbs and system downtime, e.g., for bulb replacement, suggeststhat a performance based method is preferred. However, for planningpurposes, having periodic replacements that are designed to be shorterthan the useful lifetimes of the bulbs ensure functioning UV bulbs andreduces the number of unscheduled downtimes.

The chamber window also affects the UV intensity at the substrate. SomeUV processes involve evolution of solvents or reactions that emitgaseous by-products. One example is porogen removal. Porogen is removedfrom as-deposited ultra low-k (ULK) layers with UV radiation and thegaseous by-product removed from the chamber. Sometimes the by-products,usually organic contaminants, deposit on chamber parts before they canbe removed. If deposited on the chamber window, they can build up overtime and obscure the window to reduce the UV radiation passing through.Efforts to reduce these unwanted deposits, e.g., inert gas curtain belowthe window or higher purge gas flows, have not eradicated them. Thus,even if the UV bulbs are performing well, UV intensity at the substratemay still be reduced.

One attempt to address wafer to wafer cure uniformity is to measure theUV intensity at the substrate position. A lightpipe embedded in thepedestal or substrate support can measure the UV radiation from time totime. A light pipe collects radiation at the tip, usually made ofsapphire, and transmits it through optical fibers to a photodetector.The change in the lightpipe measurement over time approximates thatexperienced by the substrate. However, the gaseous contaminants candeposit on the lightpipe and must be constantly and consistentlycleaned. The constant and high powered UV exposure can solarize thelightpipe material and change its sensitivity. Embedded in the pedestal,the lightpipe is subjected to thermal cycles with the heated pedestal,which changes the transmitted value. The heated pedestal also emitsinfrared radiation which interferes with the light measured. Finally,the optical fiber connecting from the lightpipe has also been foundsolarized, which degraded the signal.

The lightpipe is periodically recalibrated as its measurements changeover time. The lightpipe measures a radiance temperature that isaffected by the chamber environment. Changes in the environment and thelightpipe over time can affect interpretation of its measurement unlessit is periodically calibrated. As discussed above, knowing the UVintensity at the substrate position may not resolve the cause—a low UVintensity may mean bad bulb performance or dirty window or both. In oneaspect, the present invention allows a periodic calibration of thelightpipe that is unaffected by the conditions inside the chamber.

In another aspect, the present invention is a method of calibrating a UVapparatus. The measurements from the in-situ UV detector of the presentinvention may be used to calibrate the UV apparatus independently of thelightpipe. The in-situ detection and detector assembly apparatus avoidmany of the problems of the lightpipe. The in-situ detector is mountedoutside of the chamber, thus it is immune to any gaseous contaminantsdepositing on the detector. In certain embodiments, it is protected fromsolarization effects by a shutter mechanism that only opens duringmeasurement. It is protected from thermal cycling by nitrogen purge gasand shielded from pedestal-emitted infrared radiation emitted by thecalibration substrate. In some embodiments, the measured value can beused to determine the exact remedial/maintenance action required,because the effects of a dirty window can be isolated. Reflecting UVradiation to an outside UV detector is advantageous because it can bedone under normal UV cure operating conditions. A calibration substratecan withstand substrate support temperatures of 400° C. or greater andcan be inserted automatically using system controls. A calibration canimmediately follow UV processing, without waiting for the pedestal tocool or chamber to be opened. Since chamber vacuum and temperature aremaintained, time required to return the chamber to service is alsoreduced.

FIG. 1A shows a cross section schematic of one implementation of twolamp assemblies using an embodiment of the present invention. Theschematic of FIG. 1A shows a configuration while a UV measurement takesplace. Lamp assemblies are mounted over a process chamber 112 overwindow 108. Linear bulbs 102 generate UV radiation in all directions.Some of the generated radiation directly irradiates the calibrationsubstrate 110 which is supported by substrate support 114. Some of theradiation is reflected off various reflectors before irradiating thesubstrate. Overhead reflectors 104 surround the bulb and redirectradiation toward the substrate. The ring reflector 106 surrounds thespace above the process chamber window 108. A UV detector 122 ispositioned inside a detector assembly 120. As shown in FIG. 1A, lightray 116 from the bulb 102 is reflected off the calibration substrate 110to the UV detector 120. A light ray may also be reflected first from areflector before reaching the substrate, such as light ray 118. Lightrays 116 and 118 are both detected by detector 122. Note that beforereaching the detector, the light passes through the window 108 twice.

FIG. 1B shows a top view of the same lamp assembly as FIG. 1A. Lampassembly housings 124 enclose tubular bulbs 102 and overhead reflectors104 and are rectangular in shape. They are mounted over ring reflector106 which sits over the process chamber 112. The detector 122 issituated in between the lamp assembly housings and below the bulbs.

FIG. 1C shows the detector assembly 120 in the shut position. Just likein FIG. 1A, UV light reflected from the calibration substrate 110reaches the detector assembly, but note that the light does not reachthe detector 122. The detector assembly 120 has a closed cover thatprotects the UV detector from reflected radiation during, for example,UV curing of substrates. As required when a measurement takes place, thecover can then open to expose the detector to the UV radiation.

In FIG. 1D, a mirror 126 is positioned on top of the process window totake another measurement. For this measurement, only UV light reflectedfrom the mirror is measured, for example, light ray 124. Thismeasurement removes the effect of window cleanliness from the resultbecause light ray 124 does not pass through the window 108 at all beforebeing detected. As an example, light ray 128 is prevented from reachingthe detector because of the mirror 108 in its path. Thus thepass-through rate of the window is removed from this measurement.

Results from the measurement of FIG. 1A over time correspond to the UVintensity as seen by the calibration substrate. However one must notethat this intensity differs from that of the light pipe described abovebecause one more pass through the window is included. Results from themeasurement of FIG. 1D over time correspond to a condition of the UVbulbs 102. Because the effects of the window deposits are removed, thismeasurement gives a good indication whether the bulbs perform asexpected. The difference in deviation over time between, say, time A andB for the measurement of FIG. 1A and time A and B for the measurement ofFIG. 1D indicates how much the chamber window reduces UV transmission.For example, if the measurement from FIG. 1A shows a 75% decrease over atime period, but the measurement from FIG. 1D shows only a 90% decreaseover the same time period, then the additional 15% must come frompassing through the window. A trigger can then be set below which thewindow must be cleaned.

Process

In one aspect, the present invention pertains to a method to calibrate asemiconductor UV cure apparatus. FIGS. 2A and 2B are process flowdiagrams showing certain embodiments of the present invention. Thecalibration usually occurs after a triggering event. In operation 201, acalibration indication is received or that a calibration requirement isdetermined. A calibration indication may be received from an operator orthe system from a downstream process that determined an unacceptablewafer-to-wafer uniformity. An indication from an operator may beinitiated as part of preventive maintenance or as part oftrouble-shooting to resolve UV intensity loss.

Alternatively, a calibration requirement may be determined, such as thecontrol system determining that a triggering event has occurred. Atriggering event may be based on accumulation of a consumption-basedvariable, a performance-based measurement such as UV intensity from alight pipe, or an absolute number of time or substrates processed. Avariable indicating consumption of UV bulbs may be total power. UV bulbsmay have life expectancies measured in terms of total power. An exampleof an accumulation of a variable event may be total power consumptionfrom the lamp assemblies. The total power consumed may occur over manyor a few substrates processed, over a long duration or very short amountof time. This kind of indication mirrors consumption of consumable life,but is harder to keep track of or calculate than other kinds ofindications. Another kind of indication is a measurement that indicatesperformance. In this case the measurement may be UV intensity from alightpipe. A change in lightpipe measurement within a few substratesprocessed or within a few hours may indicate a change in performance ofthe UV bulbs or the window transmission. As discussed above, othervariables affect the lightpipe measurement so it alone cannot be used todetermine remedial action. The last kind of indication may be passage oftime or number of substrates processed. This kind of the indication isnot directly related to the consumption of life expectancy, but may beused instead when a consumption-based variable is hard to measure orcalculate. To illustrate, a calibration trigger may occur every 8 hoursregardless whether any substrates were processed. A calibration triggermay also occur after every 100 substrates processed regardless ofwhether the processing was for 15 minutes or for 15 seconds.

The calibration is initiated by positioning a calibration substrate on asubstrate holder, in operation 203. A calibration substrate reflects UVradiation into the detector during measurement. The reflectivity of asubstrate at various UV wavelengths is known and changes little overtime. The UV detector measurement from the same substrate can becompared and difference in UV intensities determined. The substrate maybe a bare silicon wafer or a coated silicon wafer. The coating may behigh temperature UV coating that is resistant to UV radiation and hightemperatures, for example, metal or metal oxide. Dielectric coatingsapplied with ION beam assist may be able to withstand a 400° C.operating temperature and may be suitable for this application.

In one embodiment, more than one calibration substrates having differentreflectivities in the UV wavelengths of interest are used. The intensitymeasured from two substrates may be compared to isolate the intensitymeasurement at the UV wavelength or wavelengths of interest. In a simpleexample, two otherwise identical substrates may be made, but onesubstrate may have a dielectric coating designed to absorb UV radiationat 250 nm (0 reflectivity). By comparing the intensity measured, the UVintensity at 250 nm may be isolated. This technique may be expanded tomeasure UV intensity at more than one wavelength of interest and mayeven involve more than two calibration substrates. In some UV cureprocesses, the desired reaction occurs with radiation at particularwavelengths. For those processes, knowing the intensity at a particularwavelength may be important.

When not in use, the calibration substrate may be kept in thesemiconductor processing system or outside of the system. For example,the substrate may be stored in a front opening unified pod (FOUP) closeto the semiconductor processing apparatus or in the fabrication plant'swafer storage system. Additionally, the apparatus may include a slot forstoring the calibration substrate that is accessible to the system robotarms.

The UV radiation power is set to a first power value in operation 205.This first power may be 100% power. Preferably, the UV intensitymeasurements are taken at the same power value for ease of comparison.The UV lamp may be on or off when the calibration substrate ispositioned, and in some cases the UV lamp may be already set to thefirst power value.

In certain embodiments, the UV detector is protected by an assembly thatopens and closes a cover to sequentially allow UV detection and toisolate the detector from UV radiation. This cover may be a shutter oran iris. In optional operation 207, a shutter is opened to expose the UVdetector to UV radiation. During the time the cover or shutter is open,the UV detector measures the radiation and provides an output, inoperation 209. This output is read or measured over a duration of time.Once the measurement is complete, the shutter may be closed to isolatethe UV detector from the UV radiation, in operation 211.

Operations 203 to 211 may be repeated during the calibration. Asdisclosed above, more than one calibration substrate may be used, thusrequiring a repeat of the measurement operations. The measurement mayalso be repeated using a mirror between the UV detector and chamberwindow, as further explained below in operation 219 and 221.

FIG. 2A is continued on FIG. 2B. In operation 213, a UV intensity basedon the detector output measured in operation 209 is calculated. The UVdetector output may be a current or a voltage. The current or voltage isconverted to an equivalent UV intensity, which may be watts per squarecentimeter or just a percentage of maximum. In operation 215, adeviation between the calculated UV intensity and a baseline intensityis calculated. The baseline intensity is the expected value for thepower level. Based on the deviation, an adjustment of the UV cureprocess parameters may occur. If the deviation is very small or none, noadjustment needs to be made. However, if the deviation is in a rangewhere adjusting the UV radiation power or exposure time may efficientlycompensate for the deviation, then the adjustment may be made inoperation 217. For example, if the measured intensity is higher than theexpected baseline intensity, then the UV radiation power may be reducedto affect the same UV intensity. More commonly, the measured intensityis lower than the expected baseline intensity. The preferred adjustmentmay be to increase the UV radiation power, if the power is not alreadyat the maximum. If the power is already at the maximum, then theexposure time may be increased to compensate for the deviation. Notethat increasing exposure time decreases system throughput, which isundesirable. Thus this adjustment may be made deliberately if theresulting throughput is within desirable limits.

Optional measurements using a mirror to isolate the effects of chamberwindow deposits may be made before, at the same time or after thecalculation operations and the calibration substrate measurement. Amirror may be positioned between a process chamber and a UV detector, inoperation 219. In some embodiments, this mirror positioning may beperformed mechanically by using a robot arm or a solenoid. In otherembodiments, an operator may perform this mirror positioning while theUV bulb is off. For example, a door on the side of the lamp assembly maybe opened and a mirror inserted into a slot. The mirror is highlyreflective of UV light. In certain embodiments, it may be a portion of acalibration substrate that is the same as one used inside the chamber.It other embodiments, it may be a piece of metal or glass coated withreflective material.

In operation 221, a UV intensity measurement is taken to measurereflected UV radiation from the mirror. This operation may be performedby repeating operations 205 to 211. During the measurement, whether thecalibration substrate is on the substrate holder is not importantbecause light reflected from the substrate is not measured. After the UVintensity is measured, the mirror may be removed in operation 223.Similar to the positioning operation, the mirror may be removedmechanically or by operator and automatically or manually.

As disclosed above, the UV intensity reflected from the mirror may beused to determine a cleanliness of the process window. From the windowcleanliness and the deviation calculated in operation 215, a number ofmaintenance activities may be performed. In operation 225, the activityto be performed is determined. For example, if the window cleanlinessaccounts for most of the deviation, then the window is cleaned, eitherthrough a remote plasma clean or dissembling the chamber and manuallycleaning the inside. In another example, if the deviation is not causedby window cleanliness, then the UV bulbs may be replaced. Becausedissembling the chamber requires shutting down the entire system andtakes a lot of time, it is avoided if possible. Knowing the deviationand whether the window is dirty allows the operator or the system todetermine the best maintenance activity that minimizes downtime.

Other operations may be performed in addition to those of FIGS. 2A and2B. For example, a comparison of UV intensity measured with differentcalibration substrates, as discussed above, may be done to resolve UVintensity at particular wavelengths that is important to the process.Other ways to resolve UV intensity at particular wavelengths may includeusing wavelength specific detector and adding a filter between thedetector and the window. One skilled in the art would be able to designvarious ways to measure UV intensity overall, at different bands, or atspecific wavelengths using the concepts disclosed herein.

In some embodiments, methods and assemblies that prevent or reducesensor contamination are provided. Contamination can accumulate on theface of a detector and/or window and obscure UV light. This causes adownward drift of the detected UV signal. The drift is an artifact ofthe sensor obscuration and not indicative of the UV light that reachesthe sensor face. Contamination can be caused by UV-induced decompositionof organic impurities in, for example, an inert gas flow and/oroutgassing from outgassing from potential contaminated surfaces in theproximity of the detector. UV light decomposes organic compounds exposedto it, causing hydrocarbon outgassing and condensation on opticalsurfaces of the detector. The accumulated condensed hydrocarbons absorbUV light, causing the downward drift of the detected signal.

Methods include flowing a gas mixture include molecular oxygen (O₂) overthe optical surfaces of the detector. For example, in certainembodiments, clean dry air (CDA) available from a facility source issupplied in a mixture with one more inert gases such as helium (He),argon (Ar) or nitrogen (N₂). The molecular oxygen reacts in the presenceof short wavelength (around 160-250 nm) UV to form ozone (O₃). The ozonereacts with the organic contaminants on the optical surfaces, forminggaseous products that can be purged. In some embodiments, a constantflow of a coolant gas including CDO or other O₂-containing gas is usedthrough operation. In some embodiments, the coolant gas consistsessentially of molecular oxygen and one or more inert gasses. FIG. 6 isa graph showing detected UV signal % over the course of usage of a lamp,comparing a clean N₂-only coolant gas (data series 603) and anozone-generating coolant gas (data series 601). The clean N₂ flowcontains some amount of organic contaminants as evidence by the drift insignal. The ozone purge shows no drift. The same UV light that causesthe contamination works to constantly clean the contamination andeliminate its artifact.

The amount of molecular in the gas mixture is controlled to control theamount of ozone generated. If too much ozone is present in the detectorassembly, it can absorb UV, providing inaccurate measurements of the UVintensity. It may also damage organic material in the vicinity of thedetector (such as cables and printed circuit boards). In certainembodiments, the coolant gas is actively pumped out of the detectorassembly to control the amount of ozone in the assembly. The addition ofan inert gas (such as N₂) may be used to dilute the O₂-containing gas toreduce the total amount of ozone. In addition to the amount ofO₂-containing gas, the amount of ozone present is also affected by theremoval rate of the exhausting hardware. Example flows are providedbelow, using ozone exhausting hardware capable of exhausting 40-50 SLM.

Total N₂ and CDA CDA Flow Contribution Combined Flow (SLM) (SLM) O₃Level (ppm) 150 50 =>22 100 100 =>22 50 50   6

In some embodiments, an ozone level between about 5 and 15 ppm ozone isused. In some embodiments, O₂ is about 5%-20% of the total gas flow.Accordingly, if air is used as the O₂-containing gas, the air flowcontribution between about 20% to 100% of the total gas flow. Lower O₂contributions (e.g., 1%-5%) may be used if necessary to lower the ozonelevel for the reasons described above.

Apparatus

FIGS. 3A and 3B shows more detail in one implementation of the detectorassembly from different views. In FIG. 3A, the UV detector 301 islocated inside the assembly 300. The detector 301 faces an opening orslot 305 through which reflected UV radiation may enter the assembly. Acover 303 may be moved to cover the opening or slot 305 to isolate theUV detector 301 from reflected UV radiation. During UV lamp operation,inert gases such as argon or nitrogen, at about 1-10 slm, flow into theassembly through inlet 307 to cool the assembly and to remove anyparticles that result from the moving cover. The inert gas containsnitrogen in some implementations, which may be at room temperature orcooled. The gas exits the assembly below the cover. As described furtherbelow, in some embodiments, an oxygen-containing gas is flown throughthe assembly in addition to or instead of the inert gas. For example, amixture of nitrogen and clean dry air (CDA) may be flown through theassembly.

FIG. 3B shows the assembly of FIG. 3A in a side perspective. The UVdetector 301 show also includes a diffuser 311 installed on its face.The diffuser integrates light from different directions and is known inthe art. Inert gas injected from inlet 307 exits the assembly below thecover 303. As shown, a small gap exists between the cover and the bottomof the UV detector assembly.

In the embodiment shown on FIG. 3B, a solenoid 309 rotates the cover 303in place to cover the opening 305 for isolating the detector 301 or outand away to expose the detector 301 to reflected UV radiation. In otherembodiments, other mechanical means may be used to effectuate thecovering and uncovering of the opening. For example, the cover orshutter may be moved by a lead screw or a stepper motor. The cover alsomay be an iris with multiple pieces that rotate at the same time. Asuitable mechanism is remotely controllable either electronically ormechanically from outside of the lamp assembly.

FIG. 3C is a schematic of another implementation of a detector assembly.Detector 301 faces an opening or slot 305 through which UV radiation mayenter the assembly. Gases enter at inlet 307 and flow over opticalsurfaces 315 a, 315 b and 317, exiting at outlet 319. Optical surfacesmay be lenses, windows, filters and the like. An exhaust pump 321connected to outlet 319 pumps the gases out. In some embodiments,multiple inlets and/or outlets may be arranged to establish a flow pathover any optical surface or other surface on which contamination mayoccur. FIG. 3D is a schematic of a light pipe sensor configuration,including UV detector 301. UV light enters aperture 305 and istransmitted through light pipe 330 to detector 301. Gas enters at inlet307 and exits at outlet 319.

FIG. 4 is a schematic of a process chamber suitable for practicing thepresent invention. UV lamp assemblies 401 are mounted on top of processchamber 413. Each lamp assembly 401 includes transformers 403 andmagnetrons (not shown) that pump microwave energy into the lamp, UV bulb405, and reflectors 407. As shown, the UV detector assembly 409 ismounted below and between two lamp assemblies 401. A chamber window 411transmits UV radiation from the bulbs 405 to the substrate 417 below inthe chamber. The substrate 417 sits on a substrate support 415, whichmay be heated, cooled or both. As shown, a heater coil 419 is embeddedin the substrate support 415.

In some embodiments, the process chamber includes multiple stations eachhaving a substrate support, chamber window, and UV lamp assemblies.FIGS. 5A and 5B are schematics of a multi-station chamber suitable forpracticing the present invention. Chamber 501 includes four stations503, 505, 507, and 509. Each station includes a substrate support 513.Embedded in the substrate support 513 are pins 519. Four carrier ringsare connected to a spindle 511 that rotates about an axis. When a newsubstrate is introduced into a chamber, for example, at station 507, itis positioned on a robot arm. The carrier ring lowers the substrate tothe substrate support, where it sits on top of the pins 519. Aftersubstrate processing is complete at one station, the carrier ring liftsthe substrate and rotates 90 degrees to the next station, where itlowers the substrate for processing. Thus a substrate enters the chamberat station 507 sequentially moves through each of the four stationsbefore exiting the chamber.

FIG. 5B shows a top view of the same multi-station chamber with UV lampassemblies 515 on top of each station. The UV detector assembly 517 ispositioned about the center between two lamp assemblies. Note that thelamp assemblies have a different relative orientation to the substrateas the substrate moves through each station. This different relativeorientation ensures that each circular section of the substrate isexposed to the same amount of UV radiation. Because the UV bulb istubular and substrate is round, in each station different parts of thesubstrate receive differing amounts of UV radiation. By changing therelative orientation of the bulbs to the substrate, these differencesare overcome. Generally, having four stations of varying orientation issufficient to ensure a relatively uniform UV irradiance. Of course, therelative orientations are not limited to the one shown in FIG.5B—multiple orientations of lamp axis can be chosen to optimize waferprocess uniformity. Uniform exposure may also be achieved by rotatingthe either the lamp assembly or the substrate during UV cure.

In certain embodiments, a system controller 521 is employed to controlprocess conditions during UV cure, calibration, insert and removesubstrates, etc. The controller will typically include one or morememory devices and one or more processors. The processor may include aCPU or computer, analog and/or digital input/output connections, steppermotor controller boards, etc.

In certain embodiments, the controller controls all of the activities ofthe apparatus. The system controller executes system control softwareincluding sets of instructions for controlling the timing, mixture ofgases, gas injection rate, chamber pressure, chamber temperature,substrate temperature, UV power levels, UV cooling gas and flow, remoteplasma clean, and other parameters of a particular process. Othercomputer programs stored on memory devices associated with thecontroller may be employed in some embodiments.

Typically there will be a user interface associated with controller 521.The user interface may include a display screen, graphical softwaredisplays of the apparatus and/or process conditions, and user inputdevices such as pointing devices, keyboards, touch screens, microphones,etc.

The computer program code for controlling the UV cure and calibrationprocesses can be written in any conventional computer readableprogramming language: for example, assembly language, C, C++, Pascal,Fortran or others. Compiled object code or script is executed by theprocessor to perform the tasks identified in the program.

The controller parameters relate to process conditions such as, forexample, UV power, UV cooling gas and flow rate, process duration,process gas composition and flow rates, temperature, pressure, andsubstrate temperature. These parameters are provided by the user in theform of a recipe, and may be entered utilizing the user interface. Thecontroller parameters that relate to calibration conditions may also beentered utilizing the user interface, such as the duration of exposure,the UV power setting for measurement, and location of the calibrationsubstrate. Additional parameters may be entered during setup orinstallation of the UV detector assembly, such as conversion formula ofthe UV detector output to UV intensity, deviation threshold for bulbreplacement and window cleaning, and whether UV intensities at specificwavelengths are to be determined. These additional parameters may bechanged from time to time during system maintenance.

Signals for monitoring the process may be provided by analog and/ordigital input connections of the system controller. The signals forcontrolling the process are output on the analog and digital outputconnections of the UV cure apparatus.

The system software may be designed or configured in many differentways. For example, various chamber component subroutines or controlobjects may be written to control operation of the chamber componentsnecessary to carry out the inventive UV intensity measurement andcalibration processes. Examples of programs or sections of programs forthis purpose include substrate positioning code, process gas controlcode, pressure control code, heater control code, and UV lamp controlcode.

A UV lamp control program may include code for setting power level tothe UV lamps, initial ramping up of lamp power, and final ramping downof lamp power. A substrate positioning program may include program codefor controlling chamber components that are used to load the substrateonto a pedestal or chuck. A process gas control program may include codefor controlling gas composition and flow rates and optionally forflowing gas into the chamber prior to UV cure in order to stabilize thepressure in the chamber. A pressure control program may include code forcontrolling the pressure in the chamber by regulating, e.g., a throttlevalve in the exhaust system of the chamber. A heater control program mayinclude code for controlling the current to a heating unit that is usedto heat the substrate. Alternatively, the heater control program maycontrol delivery of a heat transfer gas such as helium to the waferchuck.

Examples of chamber sensors that may be monitored during UV cure andcalibration include mass flow controllers, pressure sensors such asmanometers, thermocouples located in pedestal or chuck, and of course,UV detectors in the UV detector assembly and in the pedestal.Appropriately programmed feedback and control algorithms may be usedwith data from these sensors to maintain desired process conditions.

In one embodiment, the controller includes instructions for performingcalibration, adjustment of process parameters, and determiningmaintenance activities according to methods described above. Forexample, the instructions can specify the parameters needed to performthe UV cure and calibration. The instructions may include calculatingresults based on measurement using user-provided formulas or conversioncharts, integrating measurement results over time, e.g., integrating UVintensity measurement over the exposure time, and compare the result toa pre-determined threshold value. The result of the comparison may beused in logic sequences where an alarm or warning is generated, processparameter changed, or system is shutdown.

Suitable semiconductor processing tools may be configured with one ormore UV process chambers. Suitable semiconductor processing toolsinclude the SOLA and modified VECTOR available from Novellus Systems,Inc. of San Jose, Calif. Other suitable semiconductor processing toolsinclude the Centura and Producer available from Applied Materials, Inc.of Santa Clara, Calif.

Details of the UV lamp assembly, reflectors, process chamber window, andtemperature control on the substrate holder may be found in commonlyassigned co-pending U.S. application Ser. No. 11/688,695, titled“Multi-Station Sequential Curing of Dielectric Films”, which isincorporated herein by reference in its entirety for all purposes.

For convenience, discussion of the UV lamp assembly and in-situ UVdetector assembly focused on a two linear bulb configuration for the UVsource; however, the present invention is not so limited. For example,the in-situ UV detector would work with any number of bulbs, e.g., 3, 4,or 5, in any kind of configuration, e.g., parallel or end-to-end. Thein-situ UV detector would also work with other types of bulbs, not justlinear bulbs. As UV bulbs evolve and techniques to build UV lampassemblies advance, the use of an in-situ UV detector to monitor bulbperformance continues to apply.

While this invention has been described in terms of several embodiments,there are alterations, modifications, permutations, and substituteequivalents, which fall within the scope of this invention. It shouldalso be noted that there are many alternative ways of implementing themethods and apparatuses of the present invention. It is thereforeintended that the following appended claims be interpreted as includingall such alterations, modifications, permutations, and substituteequivalents as fall within the true spirit and scope of the presentinvention. The use of the singular in the claims does not mean “onlyone,” but rather “one or more,” unless otherwise stated in the claims.

1. An ultraviolet (UV) apparatus, the apparatus comprising: a processchamber comprising a substrate holder and a window; a UV radiationassembly external to the process chamber, the UV radiation assemblyoperable to direct UV radiation through the window towards the substrateholder; a detector assembly comprising a UV radiation detector, one ormore gas inlets and a gas outlet arranged so that a gas flow from theone or more gas inlets to the gas outlet flows over one or more opticalsurfaces of the UV radiation detector; and a controller comprisinginstructions for: flowing a gas mixture comprising an inert gas andmolecular oxygen into the one or more gas inlets during operation of theUV detector.
 2. The UV apparatus of claim 1 wherein the gas mixtureincludes air.
 3. The UV apparatus of claim 1 wherein the gas mixtureincludes air and N₂.
 4. The UV apparatus of claim 1 wherein the gasmixture consists essentially of air and an one or more inert gases. 5.The UV apparatus of claim 1 wherein molecular oxygen is between about 5%and 20% of the coolant gas.
 6. The UV apparatus of claim 1 where air isbetween about 20% to 100% of the coolant gas.
 7. The UV apparatus ofclaim 1 further comprising an exhaust pump operable to pump gas out ofthe detector assembly via the gas outlet.
 8. The UV apparatus of claim 1wherein the UV radiation assembly includes a UV source operable to emitUV radiation having wavelength of between about 160 nm and 200 nm.
 9. Anultraviolet (UV) apparatus comprising: (a) a process chamber comprisinga substrate holder and a window; and, (b) a UV radiation assemblyexternal to the process chamber, the assembly comprising one or more UVlamps, one or more reflectors operable to direct a portion of the UVradiation from the at one or more UV lamps through the window towardsthe substrate holder, a UV intensity detector positioned to receive UVradiation from the one or more UV lamps reflected through the window, agas inlet, and a gas outlet, wherein the gas inlet and gas outlet arepositioned such that a gas flow path from the gas inlet to the gasoutlet includes an optical surface of the UV intensity detector.
 10. Theultraviolet (UV) apparatus of claim 9 further comprising a coverconfigured to isolate the UV detector from radiation from one or more UVlamps.
 11. A method comprising: directing UV radiation from a UVradiation source external to a process chamber through a process chamberwindow; measuring UV radiation reflected from a substrate within theprocess chamber with a UV radiation detector located in a detectorassembly external to the process chamber; flowing a coolant gas throughdetector assembly while measuring UV radiation such that the coolant gascontacts optical surfaces of the UV radiation detector, wherein thecoolant gas comprises molecular oxygen.
 12. The method of claim 11further comprising generating ozone in the detector assembly from thecoolant gas.
 13. The method of claim 12 comprising maintaining an ozoneconcentration of about 5-15 ppm in the detector assembly duringmeasurement.
 14. The method of claim 11 wherein molecular oxygen isbetween about 5% and 20% of the coolant gas.
 15. The method of claim 11wherein the coolant gas includes air.
 16. The method of claim 15 whereair is between about 20% to 100% of the coolant gas.
 17. The method ofclaim 11 wherein flowing a coolant gas through the detector assemblyincludes pumping a gas mixture including ozone out of the detectorassembly.
 18. The method of claim 11 wherein the UV radiation includesUV radiation having a wavelength of between about 160 nm and 250 nm.